ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-13-3587-2013Modeling of daytime HONO vertical gradients during SHARP 2009WongK. W.13TsaiC.1LeferB.2GrossbergN.2StutzJ.11Department of Atmospheric and Oceanic Sciences, University of California Los Angeles, Los Angeles, CA 90095, USA2Department of Earth and Atmospheric Science, University of Houston, Houston, TX 77204-5007, USA3now at: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA0204201313735873601This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://www.atmos-chem-phys.net/13/3587/2013/acp-13-3587-2013.htmlThe full text article is available as a PDF file from https://www.atmos-chem-phys.net/13/3587/2013/acp-13-3587-2013.pdf

Nitrous acid (HONO) acts as a major precursor of the hydroxyl radical (OH) in
the urban atmospheric boundary layer in the morning and throughout the day.
Despite its importance, HONO formation mechanisms are not yet completely
understood. It is generally accepted that conversion of NO<sub>2</sub> on surfaces in
the presence of water is responsible for the formation of HONO in the
nocturnal boundary layer, although the type of surface on which the mechanism
occurs is still under debate. Recent observations of higher than expected
daytime HONO concentrations in both urban and rural areas indicate the
presence of unknown daytime HONO source(s). Various formation pathways in the
gas phase, and on aerosol and ground surfaces have been proposed to explain
the presence of daytime HONO. However, it is unclear which mechanism
dominates and, in the cases of heterogeneous mechanisms, on which surfaces
they occur.
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Vertical concentration profiles of HONO and its precursors can help in
identifying the dominant HONO formation pathways. In this study, daytime HONO
and NO<sub>2</sub> vertical profiles, measured in three different height intervals
(20–70, 70–130, and 130–300 m) in Houston, TX, during the 2009 Study
of Houston Atmospheric Radical Precursors (SHARP) are analyzed using a
one-dimensional (1-D) chemistry and transport model. Model results with
various HONO formation pathways suggested in the literature are compared to
the the daytime HONO and HONO/NO<sub>2</sub> ratios observed during SHARP. The best
agreement of HONO and HONO/NO<sub>2</sub> ratios between model and observations is
achieved by including both a photolytic source of HONO at the ground and on
the aerosol. Model sensitivity studies show that the observed diurnal
variations of the HONO/NO<sub>2</sub> ratio are not reproduced by the model if there is
only a photolytic HONO source on aerosol or in the gas phase from
NO<sub>2</sub><sup>*</sup> + H<sub>2</sub>O. Further analysis of the formation and loss pathways of HONO shows a
vertical dependence of HONO chemistry during the day. Photolytic HONO
formation at the ground is the major formation pathway in the lowest 20 m,
while a combination of gas-phase, photolytic formation on aerosol, and
vertical transport is responsible for daytime HONO between
200–300 m a.g.l. HONO removal is dominated by vertical transport below
20 m and photolysis between 200–300 m a.g.l.